1. Field of the Invention
This invention relates to a display data conversion apparatus and an LED head apparatus using the conversion apparatus. More particularly, this invention relates to a display data conversion apparatus that reduces the number of shift registers by executing display data conversion, and an LED head apparatus, for use in an LED printer using an LED array as an exposure light source for forming images.
2. Description of the Related Art
An LED printer is one of the forms of electro-photographic image formation apparatuses, and uses a photosensitive head utilizing an LED array as exposure means for optical image data relative to a photosensitive drum. The LED array is constituted as a large number of light emitting diodes (LED) are aligned in an array. In the LED printer, the LED array emits optical image data to the photosensitive drum that is charged by a charger, and an electrostatic latent image is formed on the photosensitive drum. As the photosensitive drum rotates, a toner is allowed to adhere to the electrostatic latent image by a developing device, and is transferred by a transferring device to a sheet of paper. The portion of the photosensitive drum on which the electrostatic latent image is formed is discharged by a discharging device, and a cleaner cleans the photosensitive drum to its original state. The photosensitive drum again reaches the charging device and repeats the operation described above.
Such an LED printer comprises an LED array having LED cells aligned in a predetermined pitch, a shift register having the same number of registers each for storing one image data corresponding to one LED cell, as the number of the LED cells, and an LED driving circuit for driving the LED cells to emit light in accordance with the image data stored in the register.
The arrangement pitch of the LED cells determines the printing pixel density of the LED printer. Japanese Unexamined Patent Publication (Kokai) No. 7-156422 describes an LED printer for printing image data having a plurality of different kinds of resolution by using one LED head apparatus. This LED printer uses an LED array having LED cells in a pixel pitch of resolution, that is the least common multiple of different kinds of resolution, and collectively turns ON and OFF a plurality of LED cells adjacent to one another as one pixel unit when the resolution is lower. The LED printer described in this reference prints the printing data having a plurality of different resolution by using a high-resolution LED head having a resolution of the least common multiple of these low resolution. For example, the LED printer prints four kinds of printing data having resolution of 240 dpi, 300 dpi, 400 dpi and 600 dpi by an LED head apparatus having resolution of 1,200 dpi as the least common multiple of resolution of the printing data.
Japanese Unexamined Patent Publication (Kokai) No. 7-178959 teaches means for coping with different levels of resolution by making the frequency of a transfer clock signal of image data to be given to an LED head apparatus, on the basis of a clock pulse, variable. The reference teaches to lower the frequency of the transfer clock signal when resolution is low, and to collectively turn ON and OFF a plurality of LED cells adjacent to one another as one pixel unit.
Incidentally, when the printing speed is increased in the LED printer, the frequency of the transfer clock signal to be given to the LED head apparatus becomes correspondingly higher. In order to let a LED printer print different kinds of printing data having different low degrees of resolution, the LED printer has to be equipped with a high resolution LED head apparatus having a resolution of at least the least common multiple of a plurality of such low resolutions. When a high resolution LED head apparatus is provided in the LED printer, the amount of printing pixel data transferred by a shift register increases, and transfer clock signals having a higher frequency become necessary.
For these reasons, high-speed register cells become necessary to constitute the shift register of the LED printer capable of high-speed printing. Since such high-speed shift register cells are expensive, the cost of the LED head apparatus becomes high.
For instance, the frequency F of the transfer clock signal in a continuous printer of 10,000 lines/min (approx. 0.42 cm/line) and a printing width of 43.18 cm (17 inches) can be calculated as follows.
(1) When an LED head for each resolution is used:
240 dpi: F greater than 27.2 MHz
300 dpi: F greater than 42.5 MHz
400 dpi: F greater than 75.5 MHz
(2) When three kinds of printing data of 240 dpi, 300 dpi and 400 dpi are printed by LED head apparatus of 1,200 dpi as least common multiple of these resolution:
240 dpi: F greater than 136.0 MHz
300 dpi: F greater than 170.0 MHz
400 dpi: F greater than 226.6 MHz
When one LED head apparatus prints the low-resolution printing data of a plurality of kinds in this way, the frequency F of the necessary transfer clock signal increases, and the cost of the LED apparatus becomes high.
It is therefore a first object of the present invention to provide an economical display data conversion apparatus that includes a display device of a high resolution for a plurality of different low-resolution display data, and can transfer a plurality of different low-resolution display data without increasing the frequency of a transfer clock signal of the display data when these low-resolution display data are displayed at the same resolution as their resolution, to reduce the cost of a display data conversion apparatus.
It is a second object of the present invention to provide an economical LED head apparatus that utilizes the display data conversion apparatus described above, includes a high-resolution LED head for a plurality of different low-resolution printing data, and can transfer a plurality of different low-resolution printing data without increasing the frequency of a transfer clock signal of the printing data when these low-resolution printing data re printed by one LED printer at the same resolution as their resolution, and can reduce the cost of an LED printer apparatus.
To accomplish the objects described above, a light emission control apparatus of an LED array according to the present invention is a display data conversion apparatus for causing a high-resolution display device to display input display data having a plurality of different low resolutions at the same resolution as that of the input data, the display device having a predetermined number of light emitting cells aligned in a predetermined pitch, the display data conversion apparatus comprising a switch matrix circuit, a shift register, a register stage number changing circuit, wherein the switch matrix circuit includes output terminals for outputting driving signals for driving the light emitting cells of the display device, respectively, a smaller number of input terminals than the number of the output terminals, for inputting display data, a matrix circuit for converting the display data inputted to the input terminals to display data having the same bit number as the number of the output terminals in accordance with resolution of the display data, and resolution signal input terminals for inputting a resolution signal of display data for executing the conversion operation; and wherein the shift register comprises the same number of registers as the number of the input terminals of the switch matrix circuit, each of the registers is cascaded, the output of each of the registers is connected to the input terminal of the register, and short-circuits are connected in parallel with at least one group of a predetermined number of the registers continuously connected to one another, through a select switch; and wherein the short-circuits are selected by means of the select switch in accordance with resolution of the display data, and this changes the number of stage of the registers of the shift register.
In this case, the shift register and the switch matrix circuit is constituted by cascading units each having a size which is an integral fraction of the number of light emitting cells of the light emitting device, and the shift register of each of the units has a plurality of the short-circuits so as to generate short-circuits of different numbers of the registers from the side of the rear stages.
When a plurality of kinds of the low resolutions is R1, R2 and R3, resolution of the display device is Ro, the number of the output terminals of the switch matrix circuit is Ns and the number of the registers inside the shift register is Nr, the output pixel number Ns of the switch matrix is expressed as the least common multiple of Ro/R1, Ro/R2 and Ro/R3, and the number Nr of the registers inside the shift register is expressed as the greatest one of NsR1/R0, NsR2/Ro and NsR2/Ro, and the short-circuits are disposed between the select switch and the output stage of the (NsR1/Ro)th register, between the select switch and the output stage of the (NsR2/Ro)th register and between the select switch and the (NsR2/Ro)th register. In this case, R1, R2 and R3 may be 400, 300 and 240, respectively.
In the display data conversion apparatus having the construction described above, the LED head apparatus according to the present invention can be constituted by connecting an LED array having a large number of LED cells aligned in a predetermined pitch to the post stage of the switch matrix circuit of the display data conversion apparatus through an LED driving circuit, and by converting display data having a plurality of kinds of different resolution to printing data having a plurality of different low resolution.
The present invention provides an economical display data conversion apparatus that includes a high-resolution display device for display data having a plurality of different low resolution, and can transfer a plurality of different low-resolution display data without increasing the frequency of a transfer clock signal of display data when these low-resolution display data are displayed at the same resolution as their resolution. Therefore, the present invention can reduce the cost of the display data conversion apparatus.
The present invention provides also an economical LED head apparatus that utilizes the display data conversion apparatus described above, includes a high-resolution LED head for a plurality of different low-resolution printing data and can transfer a plurality of different low-resolution printing data without increasing the frequency of a transfer clock signal of the printing data when these low-resolution printing data are printed at the same resolution as their resolution by using one LED printer. Therefore, the present invention can reduce the cost of an LED printer apparatus.